I noticed that some power AC-DC adapters with hefty amperage listed, say 2 -3 amps or so for scanners or notebook PC’s , are relatively small compared to how big and heavy a 2 amp DC power supply is that comes with less expensive stuff like a USB hub for example, but that the lighter ones tend to get hotter.
What’s the difference? Iron core vs some kind of switching PS technology? Could they make the USB hub adapter smaller and lighter if they chose to or does it need an iron core transformer?
You’ve answered your own question. The heavy ones use large, mains frequency transformers, the lighter ones use smaller transformers operating at a higher frequency (say 40 kHz), and driven by a switching device. It’s just a cost trade-off.
A mains transformer is the simplest thing you can imagine. OTOH, switching adapters or power supplies are making huge strides. The trick is they use a tiny ferrite transformer switching at much higher frequency. They have electronics 'n stuff but they are becoming competitive. Besides being lighter, another major advantage is that a switching power supply can be easily designed to accept a wide range of voltages (say 100 - 240 volts) whereas a transformer you need a switch. Forget to switch and you burn a transformer if not a laptop. So switching power supplies are very convenient for travellers for those two reasons.
They are also more efficient than linear regulated power supplies but, for a simple, single voltage, adapter, a transformer is still cheaper.
Sailor is correct. Just to expand a little: there are “classic tradeoffs” when comparing linear vs. switching power supplies.
A switching supply is usually more efficient, lighter, and occupies less volume than a comparable linear supply. Many also have wide input ranges. But there are disadvantages of switching power supplies when compared to linear supplies, namely:
But as noted by Sailor, switching supplies have “come a long way, baby.” They’re getting more reliable, and many are employing front-end “power factor correction” circuitry. Yet they’re still somewhat noisy and less stable vs. linear supplies, which is why the latter is almost exclusively used for high precision / analog applications.
<hijack>
This is off the subject a bit, but here is something I have always wondered…
Those transformers take up a ton or room on the power strip. Why doesnt someone just make some sort or universal transformer that you can plug everything that needs a transformer into?
and i’m not talking about the generic kinds you can get as a replacement. I talking about something you can plug multiple transformer-requiring items into at the same time
</hijack>
Because all devices do not have the same power needs. Some are AC, while some are DC. Some require 3 V, while some require 30 V. Some require 5 A, while some only require 5 mA. Some require a regulated supply, while some don’t care. And if you haven’t noticed, there are exactly 3 million different kind of connectors used w/ “wall wart” power supplies.
At the very least, making such a “universal / multi-channel” power supply would require that:
You would also need a dizzying array of adapter cables.
Very impractical, to say the least…
Do “switching” supplies still use transformers? Is there any reason you can’t use pulse-width modulation to drop directly from 120V to the output voltage?
In simple terms: a switching power supply charges a capacitor to the mains voltage and then uses a high frequency transformer to step it down to the desired voltage. A feedback look controls the switching frequency and/or width in order to maintain output voltage. You cannot eliminate the transformer which is needed for galvanic isolation.
Sure, you could design a power supply without a transformer. In fact, it would probably be smaller, lighter, and slightly more efficient than a similar unit w/ a transformer.
But you had better get a get a good lawyer. As noted by sailor, a transformer not only steps down the voltage, it also provides isolation from the 120 VAC outlet. If anything were to break in the power supply, the transformer would continue to provide isolation; without a transformer, it could be zap-city.